1. Field of the Invention
The present invention relates to a mold for ball grid array (BGA) semiconductor packages, and more particularly to a mold for BGA semiconductor packages which is capable of applying uniform and optimum clamping pressure to engaging surfaces of top and bottom molds upon molding a resin encapsulant in the portion of a printed circuit board (PCB) where a semiconductor chip is mounted, thereby preventing the PCB from being damaged while achieving an improvement in the quality of packages.
2. Description of the Prior Art
BGA semiconductor packages have been developed to overcome a limitation in fine pitch surface mounting. BGA semiconductor packages achieve a high integration of circuits because they can accommodate a maximum number of input/output terminals per unit area. Such BGA semiconductor packages also have a light, thin, simple and compact structure while exhibiting a superior electrical characteristic and a superior heat discharge characteristic. In addition, the BGA semiconductor packages have advantages in that they achieve a high productivity, an easy extension to multi-chip modules, and a minimized cycle from the designing step to the manufacturing step.
In this regard, BGA semiconductor packages have a high applicability obtained by their high-quality characteristic and high reliance, an easy applicability to a variety of electronic devices having a very compact size, and a high added value obtained by inexpensive manufacturing costs.
Such BGA semiconductor packages include a PCB having a semiconductor chip mounting plate at the upper surface of its central portion. Circuit patterns are formed on the upper and lower surfaces of the peripheral portion of the PCB, respectively. The circuit patterns are electrically connected to each other through via holes. A semiconductor chip is attached to the semiconductor chip mounting plate of the PCB. The circuit patterns of the PCB are also electrically connected to the circuit of the semiconductor chip by means of bonding wires. The BGA semiconductor packages also include a resin encapsulant molded to protect the semiconductor chip and bonding wires from the environment such as moisture, dust or impact, and a plurality of solder balls used as input/output terminals for electrically connecting the lower circuit pattern of the PCB to an external device.
The PCB has a multilayer structure essentially consisting of a flat, central resin layer made of a polymer resin such as polyimide or bismaleimidetriazine, signal layers comprised of metal thin films respectively formed over the upper and lower surfaces of the central resin layer while interposing coating layers therebetween, and insulating solder mask layers respectively formed over the signal layers while being externally exposed. The signal layers respectively formed over the upper and lower surfaces of the flat resin layer are electrically connected to each other through via holes. If necessary, the PCB may have at least two laminated flat resin layers on which signal layers are formed. Accordingly, the PCB has an optional thickness.
Typically, the fabrication of BGA semiconductor packages using PCB's having the above-mentioned structure is carried out in the unit of a PCB strip on which a plurality of unit PCB's are continuously arranged in such a manner that they are aligned with one another, taking into consideration the processing at each processing step and the package feeding efficiency between successive processing steps. The fabrication procedure of BGA semiconductor packages involves a semiconductor chip mounting step, a wire bonding step, a molding step, a solder ball fusing step, and a singulation step. At the semiconductor chip mounting step, semiconductor chips are mounted on respective semiconductor chip mounting plates of the unit PCB's of a PCB strip by bonding each semiconductor chip to the associated semiconductor chip mounting plate using an epoxy resin. At the wire bonding step, bond pads of each mounted semiconductor chip are bonded to conductive traces of the associated unit PCB by means of bonding wires, respectively, so that the associated semiconductor chip and PCB are electrically connected. At the molding step, a resin encapsulant is formed using a mold in order to protect each semiconductor chip and associated bonding wires from the environment. At the solder ball fusing step, a plurality of solder balls are fused as input/output terminals on the lower surface of each unit PCB. At the singulation step, the PCB strip processed at the above-mentioned steps is cut into individual unit packages.
Practically, it is very difficult for PCB strips used in the fabrication of BGA semiconductor packages to maintain a uniform thickness at all portions thereof during the package fabrication, in particular, at steps of laminating desired layers and steps of coating a solder mask layer. Each PCB strip has a relatively large thickness deviation because its portions disposed adjacent to cutting lines to be used at the cutting step have an increased thickness due to a generation of burrs, as compared to other portions. As a result, such PCB strips are problematic in that they have a relatively non-uniform flatness.
Since such PCB strips having a non-uniform flatness due to a thickness deviation exhibited among different portions thereof are used in the fabrication of BGA semiconductor packages, various problems occur. For instance, where low clamping pressure is used to engage top and bottom molds in a molding process for forming a cavity where a resin encapsulant is molded as a melted molding resin is injected into the cavity and then cured, the melted molding resin may be flashed between the top and bottom molds at a thinner portion of the PCB. On the other hand, where the clamping pressure is excessively high, a thicker portion of the PCB is severely depressed as compared to other portions of the PCB, thereby causing the PCB to be deformed. In this case, a sweeping caused by a short circuit of wires and a generation of cracks in packages may occur. Air vents may be blocked due to the deformation of the PCB. In this case, it is difficult to easily vent air left in the cavity, thereby resulting in a degradation in the quality of molded packages such as a formation of voids or blisters.
Now, a procedure for molding BGA packages in accordance with a conventional method will be described in conjunction with FIG. 33.
FIG. 33 is a schematic view illustrating a general automatic transfer type molding apparatus TMA having a conventional configuration. As shown in FIG. 33, the TMA includes a mold M comprising a top mold 10 and a bottom mold 20. A feeding unit F is disposed at a position spaced from the mold M by a desired distance. The feeding unit F serves to store PCB strips (denoted by the reference numeral 100 in FIG. 31) and to feed the stored PCB strips. The TMA also includes a transferring unit T for transferring a PCB strip stored in the feeding unit F to the mold M, a guide unit G for guiding the PCB strip during the transfer, and a receiving unit C for receiving molded PCB strips.
In the feeding unit F, PCB strips subjected to the semiconductor chip mounting and wire bonding steps are sequentially stacked. The transferring unit T transfers the PCB strips stacked in the feeding unit F one by one to the bottom mold 20 via the guide unit G. The PCB strip transferred to the bottom mold 20 is positioned in such a manner that its portions (namely, square regions indicated by dotted lines in FIG. 31), where resin encapsulants are to be formed, are received in cavities defined when the bottom mold 20 is engaged with the top mold 10, respectively. As a melted resin is injected into the cavities and then cured, resin encapsulants are formed on the PCB strip. PCB strips formed with resin encapsulants in the above-mentioned manner are sequentially stacked in the receiving unit C. Although not shown in the figures, the PCB strips stacked in the receiving unit C after the completion of the molding step are subsequently subjected to a solder ball fusing step so as to fuse solder balls as input/output terminals on the lower surface of each unit PCB. Subsequently, the PCB strips are subjected to a singulation step in order to cut each PCB strip into package units. Thus, BGA semiconductor packages are obtained.
FIG. 34 is a sectional view illustrating a typical configuration of the conventional automatic transfer type mold M. As shown in FIG. 34, a PCB strip 100, in which mounting of semiconductor chips 103 and bonding of wires 104 have been carried out, is interposed between the top and bottom molds 10 and 20 of the mold M in such a manner that the semiconductor chips 103 are positioned within cavities CA defined between the top and bottom molds 10 and 20. As a melted resin is injected into the cavities CA and then cured, resin encapsulants are formed on the PCB strip 100.
The top mold 10 includes a base 11 provided at the lower surface thereof with a longitudinally extending recess 12, a top center block 30 extending downwardly from the central portion of the lower surface of the base 11, and a top cavity insert 40 fitted in the recess 12 around the top center block 30.
The top cavity insert 40 has a plurality of clamping holes 502 respectively corresponding to a plurality of through holes 501 formed in the base 11 so that it is firmly coupled to the base 11 by clamping members B1 extending through the clamping holes 502 and through holes 501. A top drive plate DP is mounted on the top portion of the base 11 by means of a plurality of clamping members B.
Similarly to the top mold 10, the bottom mold 20 includes a base 21 provided at the upper surface thereof with a longitudinally extending recess 22, a bottom center block 30A extending upwardly from the central portion of the upper surface of the base 21, and a bottom cavity insert 40A fitted in the recess 22 around the bottom center block 30A.
The bottom center block 30A of the bottom mold 20 has a pot (not shown) adapted to melt a molding resin, and a runner (not shown) adapted as an elongated conduit for feeding the melted resin. The bottom cavity insert 40A fitted in the recess 22 around the bottom center block 30A has a plurality of concave portions 41 at the upper surface thereof.
FIG. 35 is a plan view illustrating a conventional configuration of the bottom cavity insert 40A included in the bottom mold. As shown in FIG. 35, the bottom cavity insert 40A has a base 211, a plurality of aligned concave portions 212 formed at the upper surface of the base 211 and adapted as resin encapsulant molding regions, and a plurality of clamping portions 213 extending upwardly from the upper surface of the base 211 at the peripheral edges of the concave portions 212 to have a uniform height. A PCB strip (not shown) to be subjected to a molding process is laid on a portion of the base 211 where the concave portions 212 and clamping portions 213 are formed.
When the bottom cavity insert 40A of the bottom mold is engaged with the top cavity insert (not shown) of the top mold, the concave portions 212 of the bottom cavity insert 40A form cavities which define resin encapsulant molding regions, respectively. A runner gate RG is formed at one corner of each concave portion 212 to provide a passage for injecting a melted molding resin into the associated cavity. Air vents are also formed at the remaining corners of each concave portion 212 in order to achieve a good filling of the molding resin. Each air vent AV has a predetermined width W2 and a predetermined depth D2 in order to achieve a good air ventilation while minimizing the leakage of the molding resin therethrough (FIG. 37).
FIG. 36 is a cross-sectional view taken along the line F--F of FIG. 35. Referring to FIG. 36, it can be found that the concave portions CV of the bottom cavity insert 40A are deeper than the clamping portions by a depth D1 (corresponding to the height of resin encapsulants) while the air vents AV have a depth D2.
The molding of resin encapsulants on PCB strips 100 using the TMA mentioned above in conjunction with FIG. 33 is carried out as follows. That is, when one of PCB strips 100 stacked in the feeding unit F is transferred to the bottom mold 20 in accordance with an operation of the transferring unit T, the bottom mold 20 is raised to clamp the chip-mounted and wire-bonded PCB strip 100 between the bottom and top molds 20 and 10. At this time, cavities CA are defined between the bottom and top molds 20 and 10 by the concave portions 41 of the bottom mold 20. Thereafter, a molding resin is injected into the cavities CA and then cured. Thus, the molding procedure is completed.
As shown in the enlarged portion of FIG. 34, the top cavity insert 40 is fitted in the recess 22 of the base 11 included in the top mold 10 in such a manner that its lower surface is higher than the lower surface of the base 11 by a height t, thereby forming a step. Accordingly, it is possible to prevent the PCB strip 100 laid on the bottom cavity insert 40A of the bottom mold 20 from being over-depressed when the top cavity insert 40 depresses the upper surface of the PCB strip 100.
However, such a PCB strip has a relatively large thickness deviation among different portions thereof. This is best shown in FIG. 14 and Table 2. As a result, clamping pressures respectively applied to different portions of the PCB strip 100 by the cavity inserts 40 and 40A of the clamped top and bottom molds 10 and 20 may be different. That is, non-uniform clamping pressure is applied to the PCB strip 100. This results in a plurality of locally deformed portions on a solder mask layer which is the uppermost layer of the PCB strip. Consequently, a generation of cracks or a degradation in the reliance of finally obtained semiconductor packages may occur.
Furthermore, portions of the PCB strip 100 disposed in the vicinity of the air vents AV of the bottom cavity insert 40A are not supported by the upper surface of the bottom cavity insert 40A during the application of clamping pressure to the PCB strip 100 by the top cavity insert 40. As a result, the PCB strip 100 may be severely deformed by the clamping pressure, thereby resulting in a sweeping phenomenon, namely, a short circuit of wires. In this case, the air vents AV may be partially or completely blocked, thereby resulting in a poor air ventilation of the cavities. As a result, a generation of blisters or voids may increase. When excessively high clamping pressure is applied to the PCB strip 100, the above-mentioned phenomenons are exhibited at all portions of the PCB strip 100.
FIG. 37 is a partially-enlarged plan view illustrating, in a perspective manner, a wire sweeping phenomenon generated in a semiconductor package due to a non-uniform application of clamping pressure upon molding a resin encapsulant (corresponding to an inner region indicated by the dotted line) using the conventional mold. In FIG. 37, the reference numeral 103 denotes a semiconductor chip, 102 conductive traces of a circuit pattern, and 104 bonding wires electrically connecting the conductive traces 102 to the semiconductor chip 103.
FIG. 38 is a schematic view illustrating a poor resin filling profile exhibited after molding a resin encapsulant by use of the conventional mold of FIG. 34 including the bottom cavity insert having the conventional configuration of FIG. 37. As shown in FIG. 38, portions of the PCB strip (not shown) disposed near air vents AV are in a severely deformed state due to clamping pressure applied to the PCB strip. As a result, the air vents AV may be partially or completely blocked. In this case, air left in the cavity concentrates toward air vents AV not blocked, so that venting pressure in the air vent AV increases, thereby generating voids or blisters. As a result, a poor resin filling profile is exhibited as shown in FIG. 38. That is, the quality of the finally produced package is degraded.
Where BGA semiconductor packages are fabricated using PCB strips having a structure including at least two laminated flat resin layers, on which signal layers are formed, it is difficult to adjust the height t of the step shown in the enlarged portion of FIG. 35 when the conventional mold is used. This is because the structure of the above PCB strips has a variable thickness. In this case, a PCB strip thickness less than the height t of the step results in an insufficient clamping pressure for coupling the top and bottom molds to each other. As a result, there is a serious problem in that the molding resin is flashed. On the contrary, where the PCB strip thickness is much greater than the height t of the step, the clamping pressure generated upon coupling the top and bottom molds is excessively high, thereby generating a deformation of the PCB strip or a sweeping phenomenon of bonding wires. Furthermore, the portions of the PCB strip disposed near the air vents AV of the bottom cavity insert 40A are severely deformed due to the clamping pressure from the top cavity insert 40 applied to the PCB strip because they are not supported by the upper surface of the bottom cavity insert 40A. As a result, the air vents AV may be partially or completely blocked, thereby generating voids or blisters.
Table 1 shows results obtained after resin encapsulants are molded on PCB strips having different average thicknesses, respectively, using the conventional mold of FIG. 34 in which its step has a constant height t.
TABLE 1 ______________________________________ Result of Molding Test Depending on Average Thickness of PCB Strip PCB Molded State of State of Strip Sam- Th. Resin Encapsulant Sweep- ple (mm) Void Flash Flister Deform Crack ing Results ______________________________________ 1 0.336 .largecircle. X .largecircle. .largecircle. .largecircle. .largecircle. Bad 2 0.339 .largecircle. X .largecircle. .largecircle. .largecircle. .largecircle. Bad 3 0.342 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. Good 4 0.345 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. Good 5 0.346 .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. Good 6 0.360 X .largecircle. X X .largecircle. X Bad 7 0.360 X .largecircle. X X X X Bad 8 0.363 X .largecircle. X X X X Bad ______________________________________ Not Generated: Generated: X *Molding Condition for Resin Encapsulant Step t: 0.220 mm Molding Resin Injection Pressure: 80 kg/cm.sup.2 Molding Time: 10 to 12 sec. PCB Clamping Pressure: 200 to 250 kg/cm.sup.2
As apparent from the above results, where the step between the lower surface of the top mold 10 and the lower surface of the top cavity insert 40 has a height t of 0.220 mm, the molded state of resin encapsulants is good only for PCB strips having an average thickness ranging from 0.342 mm to 0.346 mm. In this case, the PCB strips maintain a normal state. In the case of PCB strips having a thickness beyond the above-mentioned thickness range, undesirable results are obtained, as shown in Table 1.
FIG. 39 is a bottom view illustrating the top cavity insert 40 fitted in the recess 12 of the conventional top mold 10. Since the entire bottom surface including a packaging region PA of the top cavity insert 40 is roughly surface-treated in accordance with a sanding method, a molding resin outwardly leaked through the runner (not shown) of the bottom mold serving as a resin feeding conduit and air vents (not shown) may be firmly attached to the rough surface of the top cavity insert 40 during the molding process. For this reason, it may be difficult to separate a molded PCB strip 100 from the mold. As a result, the quality of finally obtained packages and the process efficiency are degraded.
Moreover, since the top cavity insert 40 has a configuration integral with the top center block 30, it is difficult to replace the top cavity insert 40 when the top cavity insert 40 is damaged or abraded at its portion corresponding to the packaging region PA. In this case, there is also a problem in that the top mold should be completely replaced by a new one. This results in an increase in costs.
FIG. 40 is an exploded perspective view of top and bottom molds constituting another conventional mold which has a manual transfer-type configuration. In this mold denoted by the reference character M, loading bars 330 are mounted on the bottom mold denoted by the reference numeral 20. A cavity plate 350 provided with cavities for molding resin encapsulants is mounted on each loading bar 330. The top mold, which is denoted by the reference numeral 10, is coupled to the bottom mold 20. The top mold 10 is formed with packaging regions PA.
FIG. 41 is an exploded perspective view of a PCB strip interposed between the loading bar and cavity plate in the conventional mold of FIG. 40 upon its molding. FIG. 42 is a lateral sectional view illustrating a loaded state of the PCB strip shown in FIG. 41.
As shown in FIGS. 41 and 42, the cavity plate 350 defines 4 sides of resin encapsulant molding regions on the PCB strip 100. The loading bar 330 is mounted in a recess 12A of the bottom mold 20. A plurality of uniformly space pins 332 are arranged on the loading bar in such a manner that they are longitudinally aligned with one another. The PCB strip 100 and cavity plate 350 have holes 106 and 351 corresponding in position to the pins 332, respectively. By such a configuration, the PCB strip 100 and cavity plate 350 can be sequentially mounted on the loading bar 330 in a manual manner. The basic configuration of this mold is identical to that of FIG. 34.
In the case of the conventional mold M having the configuration of FIG. 40, the loading bar 330, on which the PCB strip 100 and cavity plate 350 have been mounted, is manually mounted on the bottom mold 20. Thereafter, the bottom mold 20 is raised to be coupled to the top mold 10. In this state, a molding resin is supplied to the cavities defined in the cavity plate 350 and then cured. Thus, molding of resin encapsulants having a desired shape is completed.
In this case, however, it is difficult to manually mount the loading bar 330 on the bottom mold 20 because the loading bar 330 is very heavy due to its thick plate type structure. Since the pins 332 are integral with the loading bar 330, they may be easily damaged upon manually mounting the loading bar 330 on the bottom mold 20. In this case, there is an 20 inconvenience in that the loading bar 330 should be re-machined. Furthermore, a higher clamping pressure is required upon coupling the molds 10 and 20 because of the use of the cavity plate 350. This may result in a deformation of the loading bar 330. Accordingly, a degradation in the stability occurs in the molding of packages.
Where the diameter of the holes 351 formed in the cavity plate 350 is unallowably larger than the diameter of the pins 332 inserted in the holes 351, it is impossible to obtain an accurate setting position, thereby resulting in a degradation in the quality of finally produced packages. Where the pins 332 and holes 351 have substantially identical diameters in order to solve the above-mentioned problem, it may be necessary to forcibly insert the pins 332 in the holes 351. In this case, it is difficult to easily separate the cavity plate 350 from the loading bar 330. Moreover, the pins 332 may be easily damaged.